There should be a config file in ~/.config/MangoHud (you can check more details in the GitHub page https://github.com/flightlessmango/MangoHud#hud-configuration).
By adding frametime=0 to the config file, you should be able to remove the frametime measurement from the FPS display.
It's maintained on GitHub, so you might be able to manually install it or find a package for it in your package manager. At least on Arch you can install gnome-shell-extension-ddterm-git from the AUR.
I use ddterm, which is pretty much the only remaining drop-down terminal extension. https://extensions.gnome.org/extension/3780/ddterm/
I get the same errors about descriptor, but no mention of mlt or missing libraries.
I tried the flatpak version and that seems to be working.